Is it possible that Earth still contains some elements that are yet to be found?—Jason

The periodic table contains more than a hundred chemical elements, the basic building blocks of the everything around us — living and non-living.

The atomic number of an element is determined by how many protons are found in the nucleus of an atom of that element.

Some of the elements are well-known, such as hydrogen (1), oxygen (8) and carbon (6), while are less so; seaborgium (106), flerovium (114) and darmstadtium (110). More than three-quarters of the elements on the periodic table exist naturally on Earth or elsewhere in the Universe.

The last naturally occurring element to be discovered was francium (87) in 1939. Since that discovery, plutonium (94), neptunium (93) and astatine (85), which were initially created in the lab in 1940, have since been found in nature.

The only elements left to discover fall into the super-heavy category — elements that contain more than 104 protons — says Dr Elizabeth Williams, a nuclear physicist at the Australian National University.

But it is unlikely we will discover any new naturally occurring super-heavy elements on Earth, says Williams.

She says two things are needed in order for new naturally occurring super-heavy elements to be discovered.

"First, there would have to be a natural process that would produce these elements, and secondly the elements would have to live long enough (and in sufficient quantity) for us to detect their existence.

However, she says, synthesising super-heavy elements in the lab can help scientists better understand the properties of these elements and how they are created, which then helps them understand if and how any more naturally occurring elements may be found.

"Based on what we know about how to create heavy elements in a lab, this natural process would have to be pretty extreme, and also fairly common, for us to detect a new element in our environment."

"But it is quite possible that some of the elements we have synthesised here on Earth may also be created out in the Universe, in more extreme environments than those we find here on Earth," says Williams.

"The way we typically create these elements is that we have a lighter atom, say calcium, and we accelerate that and smash it into a foil made up of heavier atoms," says Williams.

"If the accelerated atom is going fast enough, it is possible that its nucleus — the compact core of protons and neutrons at its centre — might make contact with the nucleus of one of the heavier atoms. If this happens, there is a chance that they will fuse together to form a heavier element."

While a sheet of foil might look solid to the human eye, it's much different on the atomic scale. The nucleus takes up only a tiny fraction of the space an atom occupies. To create a new element, the nuclei from each atom must collide and fuse.

"If you think of an atom being the size of the Melbourne Cricket Ground (approximately 170 metres in diameter), the nucleus is a small grape at the centre of that," she says.

But that's not all.

If the two nuclei are headed for a collision, they must overcome a strong repulsive force that opposes their fusion into a new element. This is known as the electrostatic, or Coulomb, force. Only if the nuclei hit with enough energy will they overcome this force and fuse together.

Island of stability

Williams and her colleagues are currently looking at new ways of creating new heavy elements.

"One of the things we do know is even very small changes in the ways that we try to produce these elements have a dramatic influence on the likelihood that a new element is created," says Williams.

"For example, how fast we accelerate the beam particle, what we choose in terms of the light and heavy atoms that we collide together, that can all have an effect on the probability of creating a new superheavy element."

Currently, some of the heavier synthetic elements can last several milliseconds, and then decay into smaller elements, giving off alpha particles, photons and other decay products.

But nuclear scientists are also exploring the possibility there is a group of heavy elements that live much longer than those being created today. They call this group 'The island of stability'.

"Nuclei in the island of stability are close to what nuclear physicists call 'magic numbers' of protons and neutrons," says Williams.

"Nuclei with these special numbers of protons or neutrons are more tightly bound together than surrounding nuclei. This means they tend to live longer than their immediate neighbours."

It is predicted that these relatively stable heavy elements could last several minutes, perhaps even longer.

It's up to Williams and her colleagues to determine which combination of protons and neutrons leads them to the island.

"We've predicted that there is going to be a region of relative stability around 114 or 120 [protons]. Some say there's a relatively stable neutron number around 184," says Williams.

"If we can get closer to these numbers then we should be able to do pretty well, but it's hard to predict the science."

Dr Elizabeth Williams was interviewed by Darren Osborne. She is a Research Fellow in Department of Nuclear Physics at the Australian National University. Her focus is on exploring how to create new superheavy elements.